The present invention relates to a method of and apparatus for bidirectionally transmitting a pulse signal by using a unidirectional semiconductor switch.
As a method of bidirectionally transmitting a pulse signal using a unidirectional semiconductor switch, there is known a method as disclosed in Japanese Patent Laid-open Publication JP-A-60-31312 whereby a pulse signal is transmitted while applying a negative voltage sufficiently larger than the peak value of the pulse signal to the cathode of a diode or thyristor. According to this method, a pulse signal current always flows as the forward current of the diode or thyristor, thereby allowing to bidirectionally transmit a pulse signal by using a single unidirectional semiconductor switch. There is known another method as disclosed in Japanese Patent Laid-open Publication JP-A-56-90625 wherein there is used a diode or thyristor having a large reverse recovery current. According to this method, a pulse signal is caused to flow in the reverse direction during the period while the reverse recovery current flows, thereby allowing to bidirectionally transmit a pulse signal by using a single unidirectional semiconductor switch.
The former method is associated with a problem that a positive pulse of the pulse signal has waveform distortion. The reason for this is that since the former method provides an offset current larger than the peak value of the pulse current to allow all positive and negative pulses to flow in the same direction, a maximum current passing through the semiconductor switch becomes large and therefore the region of poor linearity within the voltage-current characteristic of the semiconductor switch is inevitably used. In order to solve this problem, it is necessary to make the conduction area of a semiconductor switch unnecessarily large. The latter method is associated with a problem that a negative pulse of the pulse signal has waveform distortion. The reason for this is that if the pulse current flowing through a semiconductor switch in the reverse direction is larger than the reverse recovery current of the semiconductor switch, the pulse current is limited by the reverse recovery current and the actual current allowed to flow becomes smaller than the desired value.